Why don't people do this?

Two more reasons: In an involved debate, if I have three different plausible interpretations of what they said, I don't want to put in the time and effort to respond to all of them. It might also come across as patronising if I did, like "yes, obviously what I said was not literally true, do you think I'm stupid or something?"

Under low friction, getting clarification before responding is good advice. In high friction, I might say "I think you meant X, but I'm not sure. If you did, then A." A more subtle variant would be to attempt to summarise X and respond to it, and if X is not what they meant then they hopefully catch this. But these are still potentially wasteful and patronising if I get it wrong.